Sodastream fizz loss mystery

Sodastream sells a device that will charge a 1 liter bottle of water with CO2 gas so you can drink fizzy beverages. I always use 40F cold water from the fridge to fill the bottle, and seal it tightly, but I find that after only 1-2 days 90% of the fizz is gone, a much shorter life than fizz from 2 liter factory soda bottles that have been opened, the cap retightened, and kept in the fridge.

What I initially thought is that there must be a leak, but I submerged the bottle in the sink, shook it (with some CO2 still forming visible bubbles inside), and there were no bubbles at all from the cap or anywhere else, after 30 minutes.

So the only idea I have left has to due with the city water, which has a pH of about 9.3 (limestone surface rock). Factory soda is presumably made with initially neutral water, which becomes acidic from the CO2, but in this case, it's starting out with solutes present (bicarbonate I'm guessing, calcium, etc.) and the higher pH.

So, is it possible that during that 1-2 days when the fizz goes away, the pH of the water is dropping and the CO2 is being bound into some solute particle? I take it a system with carbonic acid and bicarbonate is a "buffer system" but I'm not sure I fully understand what would happen in my sodastream bottle.

Your hypothesis appears to be that the carbon dioxide is still in solution in the form of carbonate.

Why not test this hypothesis? Add some strong acid to the water to expel the carbonate, and see how much gas you get.

Another experiment you can do is to weigh the bottle immediately before and after carbonation, which will let you determine the amount of carbon dioxide that has dissolved. Then weigh it again a few days later to see whether the CO2 has escaped or not.

I suspect what is really happening here is that the initial amount of CO2 added to the soda is no where near the same amount as is added to commercially made sodas. Commercially prepared sodas are carbonated by pumping the water at over 100 psi into a carbonator chamber filled with many cubic feet of compressed CO2 making for a much more saturated blend of gas to water. Additionally, when carbonating water it must be as close to freezing as possible say 33º for the CO2 to stay in solution, this is why soda fountains all use a cold plate cooled under the box of ice. Lastly, for good results, using those tiny gas containers is not ever going to produce more than mediocre soda.

I suspect what is really happening here is that the initial amount of CO2 added to the soda is no where near the same amount as is added to commercially made sodas. Commercially prepared sodas are carbonated by pumping the water at over 100 psi into a carbonator chamber filled with many cubic feet of compressed CO2 making for a much more saturated blend of gas to water. Additionally, when carbonating water it must be as close to freezing as possible say 33º for the CO2 to stay in solution, this is why soda fountains all use a cold plate cooled under the box of ice. Lastly, for good results, using those tiny gas containers is not ever going to produce more than mediocre soda.

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I considered that possibility as well, but rejected it because when I scoured the web for reviews, several people reported that their soda would remain fully fizzed for a least a week. Assuming they're not lying, that would tend to exculpate the sodastream machine itself, leaving just the bottle and water as the variables that differ.

I considered that possibility as well, but rejected it because when I scoured the web for reviews, several people reported that their soda would remain fully fizzed for a least a week. Assuming they're not lying, that would tend to exculpate the sodastream machine itself, leaving just the bottle and water as the variables that differ.

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Yeah - either your machine is somehow different from theirs (weaker seals perhaps?), the storage conditions are different (your bottle leaks more than theirs), or the reviews are more wishful thinking than fact (a product review online is not exactly a scientific test).

Note: a reviewer does not have to be actually lying to report bad data.
This is why we like to insist on controlled experiments in science.

Have you tried looking for reliable data?

According to the Sodastream FAQ:The hermetic seal on the cap keeps your soda from going flat as quickly as ordinary store-bought beverages. If you make sure to close the cap tightly, and keep the bottle in the refrigerator, your SodaStream soda should remain fizzy and fresh for over a week.

... if you do not get that performance, says the manufacturer, then perhaps the seal on your cap is faulty or you don't screw them down enough? Try a newer bottle.

Note: product reviews in line with manufacturer claims are likely.
I have not found anyone testing the claims skeptically... which is what you want.